Tomahawk

The developers of Tomahawk bill their application as a music
player that fundamentally changes the way music is consumed and shared.
This application hooks you up with your friends — connect with your
Jabber /Google Talk / Twitter friends and share collections and
playlists.

Tomahawk makes use of 'resolvers' - small scripts that let
users use a variety of services, such as hooking up your Google /
Jabber account so that you can play music from other people's
libraries. This application is written in Qt.